In order to protect an electrically operated device from losses of and fluctuations in line voltage supplying the device, the device should remain in an "off" mode for a predetermined time before being restarted after a power outage. For example, in a thermostat-controlled air conditioning system, line voltage fluctuations and thermostat switch bounces may cause excessive cycling of the system compressor. Further, the compressor may be re-started after a power outage before refrigerant pressure has been equalized across the compressor. Therefore, a timed-off period (e.g., five minutes) is required before the compressor is re-started after being turned off.
According to prior practice, a resistance capacitance (RC) circuit is used to provide timed-off control. The discharge rate of the capacitor after a power outage determines the time that the compressor remains in an "off" mode before being re-started. A voltage monitoring device (e.g., a bipolar transistor) is typically used to measure the voltage across the resistor in the RC circuit. When the measured voltage drops to a predetermined level, the voltage measuring device will allow the compressor to be re-started.
One problem associated with such prior art timed-off control apparatus is that the voltage monitoring device typically draws electrical current, which distorts the measured voltage drop across the resistor. For example, when a bipolar transistor is used to monitor voltage, a certain amount of electric current is used as biasing current for the bipolar transistor. This biasing current affects the discharge rate of the capacitor, such that the timed-off control apparatus does not provide a consistent and predictable timed-off period.
There is therefore a need for an improved timed-off control apparatus and method which provides a consistent and predictable timed-off period. There is also a need for an improved timed-off control apparatus and method which provides electric current for thermostat anticipation during timed-off periods.